1. Field of the Invention
The present invention relates to a bipolar transistor and, more particularly, to a high-frequency high-output transistor.
2. Description of the Related Art
To acquire a large output from a high-frequency high-output transistor, the large current is controlled by arranging multiple unit transistors in parallel to each other and the density of currents flowing through the individual unit transistors is made uniform to prevent thermal breakdown otherwise caused by the local concentration of the current.
FIG. 1A is a plan view of a semiconductor chip exemplifying a conventional bipolar transistor, FIG. 1B is an enlarged cross-sectional view along the line B--B' in FIG. 1A, and FIG. 1C is an enlarged cross-sectional view along the line C--C' in FIG. 1A.
As shown in FIGS. 1A through 1C, a p-type base region 2 is formed on one major surface of an n-type silicon substrate 1. An insulating film 3 is formed on the surface of this base region 2. The insulating film 3 on the base region 2 is selectively etched to form stripe-shaped emitter contact holes 5 and base contact holes 4 arranged alternately. A polycrystalline silicon (polysilicon) layer 6 containing an n-type impurity is provided in each contact hole 5. The impurity in the polysilicon layer 6 is diffused to the surface of the base region 2, forming an n.sup.+ -type emitter region 7. A diffusion layer 12 made of a polysilicon layer is formed on the insulating film 3 adjacent to the base region 2. The diffusion layer 12 has a shape narrow in the center and wide at both ends. Formed in an insulating film 8 that covers the diffusion layer 12 are stripe-shaped contact holes 9. The contact holes 9 are short in the center portion of the diffusion layer 12 and become longer toward both end portions thereof. A metal film made of aluminum or the like is deposited on the surface of the resultant structure, including the polysilicon layers 6, the base contact holes 4 and the contact holes 9, and is then patterned, yielding a comb-shaped base electrode 11 which contacts the base region 2 in the base contact hole 4. Further formed are first emitter electrodes 13 which connect the polysilicon layers 6 to the resistor layer 12 under some contact holes 9. A comb-shaped second emitter electrode 14 is formed which connects to the resistor layer 12 in the other contact holes 9 than those connected with the emitter electrodes 13. The emitter regions 7 are connected to the second emitter electrode 14 via the first emitter electrodes 13 and ballast resistors whose resistances are determined by the width of the resistor layer 12 and the size of the contact holes 9. More specifically, the intervening ballast resistors have a high resistance in the center portion and a low resistance at the peripheral portion. This particular connection of the emitter regions 7 suppresses the current flowing in the center portion of the transistor, thus accomplishing the uniform operation of the unit transistors.
Since the conventional bipolar transistor has a ballast resistor connected in series to the associated emitter electrode in order to suppress the local current concentration on the center portion of the transistor, the forward transfer gain is undesirably decreased.
The formation of the ballast resistor requires a large area for forming the resistor layer in a region adjacent to the base region. This reduces the ratio of the effective utilization area of the semiconductor chip and needs an additional step for forming the ballast resistor, thus increasing the manufacturing cost.